In logging procedures, a common mode of irradiation is bombardment of the formations adjacent to the borehole with neutrons generated by a pulsed neutron tube. A pulsed neutron tube is operated periodically to form the neutron flux for obtaining an output signal at a radiation detector located in the logging tool. The neutron generator and the circuitry associated with it must be operated at relatively high temperatures typically encountered in downhole conditions. These high temperatures cause thermal drift for transistorized circuitry. Accordingly, it is difficult to obtain a stable pulsed ion source high voltage supply for operation of the neutron generator which is capable of adjustment to form pulses at varying lengths and at different frequencies. It is desirable to operate the neutron generator at different times with different pulse rates and different pulse widths. Thus, the width of the pulses may vary by perhaps ten fold, and the pulse repetition rate of frequency may vary also by ten fold.
The apparatus of this circuit is an ion source high voltage power supply capable of being pulsed at different pulse widths and different frequencies. The circuit disclosed herein is particularly deirable because it is temperature stable (transistor impedance increase with temperature constrains thermal runaway), an advantage over circuits utilizing bipolar output transistors. The change in temperature from ambient temperatures at the surface to temperatures encountered in deep wells initiates drift or even thermal runaway in bipolar transistors and hence renders such high voltage power supplies inoperative. Accordingly, this circuit is able to adjust over a range to provide wideband tuning of pulse width and frequency. Moreover, the circuit will operate at ambient temperatures and the high temperatures normally encountered in downhole conditions. For this reason, the pulsed neutron generator can then be operated at any desirable pattern for the neutron flux formed by the generator.
One advantage of this apparatus is a wide range of tuning. That is, the device is able to form pulses of different widths. The width adjustment accommodates a large range of pulse widths; in fact, there is a wide range of pulse widths permitted in the circuit itself, and practicalities limit pulse width. Likewise, pulse frequency can be varied widely and is limited only by the practicalities of application.
Another advantage of this device is the utilization of an interlocking system whereby a string of FETs connected between the high voltage output terminal and ground has FETs grouped into two groups. The two groups are switched with a timing sequence between the groups to thereby assure proper overlap. The overlap in timing protects the FET string against the unwanted circumstance where they might form a short between high voltage and ground, thereby burning up all the output transistors. Moreover, while several FETs are included in this string, control can be exerted by less than all of the transistors; that is, the control pulses are applied to less than all the output transistors and the remaining transistors are switched by a cascade switching sequence across the string.
These and other advantages will be observed on a review of the disclosed high voltage power supply capable of forming pulses of any width at any frequency for a pulsed neutron generator source. While the apparatus has been described only briefly herein, a greater understanding thereof will be obtained on review of the detailed description of the preferred embodiment set forth below which apparatus accomplishes the objects described herein and has the advantages noted above as well as other advantages.